Dinitrotoluene (DNT) is conventionally manufactured by nitrating toluene with a mixture of nitric and sulfuric acids. The crude dinitrotoluene thus obtained contains the desired 2,4- and 2,6-dinitrotoluene isomers along with small amounts, usually less than 5%, of isomeric dinitrotoluene impurities, notably the 2,3-, 2,5- and 3,4-dinitrotoluene isomers (so-called "meta" isomers). Dinitrotoluene is particularly valuable as an intermediate in the preparation of toluene diisocyanates employed in the manufacture of polyurethane foams and elastomers. In the preparation of products of this type, it is highly important to exclude the so-called "meta" isomers, since they contribute to the production of off-color, yellow polyurethane products. Dinitrotoluene is also valuable as an intermediate in the manufacture of 2,4,6-trinitroluene (TNT) employed as an explosive for military use. The TNT obtained by nitration of crude dinitrotoluene contains the desired 2,4,6-TNT isomer together with the undesired 2,4,5-, 2,3,4, and 2,3,6-TNT isomers, which result from the nitration of the aforesaid "meta" isomers present in crude dinitrotoluene. At present TNT is purified by treatment with a hot solution of sodium sulfite (sellite) solution, which removes the undesired TNT isomers but results in a very concentrated and intensely reddish colored solution, whose disposal is costly and contributes to a serious pollution of streams into which it is discharged.
It is thus evident that the elimination of the so-called "meta" isomers in crude dinitrotoluene is highly desirable, since it would permit the production of superior polyurethane products and allow the manufacture of pure 2,4,6-TNT, thereby eliminating the need for sellite purification and attendant purification problems.